engine: heat in a fluid step

A fluid domain takes `thermal` settings (the initial temperature, a
thermal condition per box face and per obstacle face, gravity with the
expansion coefficient for buoyancy) in model.fluid_domain.create and
.edit, and model.fluid_domain.thermal_boundary sets one boundary's
condition as model.fluid_domain.boundary does for the flow. A material
a domain with heat uses keeps needing its conductivity and specific
heat when it is replaced.

solve.fluid's summary adds the thermal relaxation time, the fluid's
temperature range and the heat into every wall (final and the mean
over the last quarter of the history); solve.probe gives the
temperature in the cells at points and, with `heat`, the walls' heat
histories. The form, the catalogue and docs/features.md follow.

Part of #157.

docs/features.md

@@ -108,7 +108,7 @@ the control channel and AI agents over MCP (docs/mcp.md) run the same commands.
 | Boundary Conditions | Boundary Condition — Fixed, displacement or symmetry condition on a set or reference point | ready | `model.boundary_condition.create`, `model.boundary_condition.edit`, `model.boundary_condition.delete`, `model.boundary_condition.symmetry` |
 | Loads | Load — Force, moment, pressure, traction, gravity or body force | ready | `model.load.create`, `model.load.edit`, `model.load.delete` |
 | Flow | Fluid Domain — The box of fluid cells, its fluid, and the obstacle bodies voxelised from their face-tagged surfaces | ready | `model.fluid_domain.create`, `model.fluid_domain.edit`, `model.fluid_domain.delete` |
-| Flow | Inlet & Outlet — Velocity inlets, pressure outlets, walls, moving and slip walls, periodic faces | ready | `model.fluid_domain.boundary` |
+| Flow | Inlet & Outlet — Velocity inlets, pressure outlets, walls, moving and slip walls, periodic faces; held temperatures, heat fluxes and adiabatic faces | ready | `model.fluid_domain.boundary`, `model.fluid_domain.thermal_boundary` |
 
 ## Interactions
 
@@ -141,7 +141,7 @@ the control channel and AI agents over MCP (docs/mcp.md) run the same commands.
 | Implicit | Frequency — Natural frequencies and mode shapes of a frequency step | ready | `solve.frequency` |
 | Explicit | Explicit — Transient dynamics of an explicit step by central differences (contact and erosion coming in M10) | ready | `solve.explicit` |
 | Explicit | Blast & Impact — EOS-driven fluids with fluid–structure contact (ALE or SPH) | coming soon (M14) |  |
-| Fluids | Lattice Boltzmann — Lattice Boltzmann flow of a fluid step on the CPU, with wall forces | ready | `solve.fluid` |
+| Fluids | Lattice Boltzmann — Lattice Boltzmann flow of a fluid step on the CPU or a GPU, with wall forces, and the heat it carries | ready | `solve.fluid` |
 | Fluids | Fluid–Structure — Coupled fluid and structure solve | coming soon (M13) |  |
 
 ## Results